docs: def infer_delimiter(filepath):
      """
      From extension infer delimiter used in a separated values file.
  
      :param str filepath: path to file about which to make inference
      :return str | NoneType: extension if inference succeeded; else null
      """
      ext = os.path.splitext(filepath)[1][1:].lower()
      return {"txt": "\t", "tsv": "\t", "csv": ","}.get(ext)